The present invention relates to an optical fiber star coupler by which a light beam transmitted through an optical fiber is divided to a plurality of optical fibers, and a method of manufacturing the optical fiber star coupler. It is well suited to form a star coupler by the use of plastic optical fibers.
With the rapid progress of optical fiber transmission technology, there have been vigorously made researches on and developments of optical data links which use optical fibers for the data transmission between electronic computers and between an electronic computer and a terminal. In constructing the optical data links, the optical star coupler which can mix optical signals from a plurality of input optical fibers and then divide them to a plurality of output optical fibers with low loss and equally is an indispensable device.
Heretofore, a typical example of the optical star coupler has been a biconically tapered type shown in FIG. 4. Such biconically tapered type optical star coupler is shown, for instance, in (i) "Optical Communication Handbook" edited by Hisayoshi Yanai and issued by Asakura-Shoten on Sept. 1, 1982, pages 324 and 325, (ii) T. Ozeki et al.: Electronics Letters, Vol. 12, No. 6 (1976), pages 151 and 152, and (iii) E. G. Rawson et al.: Electronics Letters, Vol. 14, No. 9 (1978), pages 274 and 275. Accordingly, it is well known.
In this optical star coupler, a multiplicity of optical fibers 1 are bundled together, and the middle part of the bundle is pulled under twisting while being heated by a heating source, to form a biconically tapered region 2, whereby optical signals from the input optical fibers (the left side of the tapered region 2) are divided to a plurality of output optical fibers (the right side of the tapered region 2).
As regards the prior-art optical fiber star coupler, there are reports in the case where the optical fibers are of a glass material, but there is no report in the case where a plastic material is employed. In view of the results of various experiments and trial manufacture conducted by the inventors of the present invention, the reasons will be ascribable to problems as stated below.
As the first reason, it has been revealed that, since plastic optical fibers have a low melting point (about 100 .degree. C.) and react very sensitively to temperatures, they are liable to unnecessary deformations or breakage attributed to fusion, to incur the problem of thermal workability. Therefore, even when the plastic fibers are intended to be twisted and pulled under heating so as to attain a desired biconically tapered shape, they fuse and rupture instantly at an intermediate stage. It has accordingly been difficult to work the plastic fibers into the biconically tapered shape which achieves an equally dividing characteristic.
As the second reason, in general, the plastic optical fiber has a large diameter (usually, an outside diameter of 0.5-1.0 mm) as shown in FIGS. 13A and 13B, and the refractive index difference between a core 3 and a cladding 4, .DELTA.n ##EQU1## n.sub.1 : refractive index of the core, n.sub.2 : refractive index of the cladding) is about 6%, which is much greater than the index difference of a silica glass optical fiber. Therefore, the degree of power concentration in the core is very high. The value will be calculated by way of trial. The degree of power concentration P.sub.c in the core is expressed by: ##EQU2## a: core diameter .lambda.: wavelength
Assuming a=730 .mu.m, P.sub.c =99.93% is evaluated, and most light is confined within the core and is thus propagated. In addition, though the thickness of the cladding is thin in the case of the plastic fiber, it is usually as great as about 10 .mu.m. For this reason, in order to realize the optical star coupler of the equally dividing characteristic by bundling such fibers, a core mode needed to be converted to a cladding mode at a stretch by increasing the number of times of twisting and shortening the pitch of twisting in the extreme. It has been revealed, however, that when the number of times of twisting is increased and the pitch of twisting is shortened in this manner, the converting from the core mode to the cladding mode involves a radiation mode in a very large proportion, which increases a radiation loss, resulting in an optical star coupler of heavy excess loss (for example, about 5 dB in a 2.times.2 port type optical star coupler having two input ports and two output ports). It has also been revealed that, when the optical fibers are pulled after or while being twisted, the individual optical fibers are not pulled uniformly, so losses fluctuate greatly between the ports. Especially in a case where the number of the fibers is large, the pulling lengths of the fiber near the central part of the fiber bundle and the fiber near the peripheral part thereof become unequal. This has been attributed to the fact that, since the fiber has the large outside diameter, the fibers bundled and then twisted come to have a very large outside diameter, so the difference of the heating temperatures of the peripheral part and the central part arises. When the pulling lengths of the individual fibers are unequal in this manner, the amount of converting from the core mode to the cladding mode in the tapered portion on the input side becomes ununiform. Besides, the amount of converting from the cladding mode to the core mode in the tapered portion on the output side becomes ununiform. As a result, the loss fluctuations between the respective ports become great.